Transmission of signals over optical fiber has become more common in part due to high signal capacity of the fiber. For example, dense wavelength division multiplexing (DWDM) systems use multiple optical wavelengths to transmit a large number of signals over a single optical fiber. Long haul optical systems allow transmission over large distances to provide signals to geographically remote locations. High value analog channels such as premium cable channels and pay-per-view can be distributed by cable providers to subscriber locations over such systems by analog modulation of the optical carriers at the different wavelengths. No digitization of the analog signals is required. Security measures are generally not employed and the opportunity for theft of the analog signals is significant.
Physical access to the optical fiber at any point along its length allows a motivated attacker to tap the fiber and eavesdrop on the transmitted channels. To tap the optical fiber, the attacker removes at least some of the cladding and bends the fiber to gain access to a portion of the optical signals that escape through the disturbed cladding. If the optical power of the tapped optical signals is sufficient, an optical receiver and associated optical and electronic components can be used to capture, or copy, a transmitted optical signal. Moreover, depending on the complexity of the equipment used by the intruder, multiple channels can be stolen. If the tapped optical signal power is small relative to the total optical signal power, subscribers are unaffected and the distribution company cannot readily detect the theft. Long haul systems are particularly vulnerable as the optical fiber length provides more opportunity for physical access.
Mechanisms currently exist to protect analog signals for transmission over an optical fiber. The analog signal can be digitized and conventional encryption techniques can be applied to the resulting data stream. Conventional encryption technology includes the use of encryption protocols such as advanced encryption standard (AES) and digital encryption standard (DES or triple-DES). Encryption protocols are generally complex and require significant processing power. Moreover, extensive hardware is required because the analog signals are converted from analog to digital format and encrypted at the transmitter and then converted from digital to analog format and decrypted at the receiver. This complexity eliminates the current advantage of simplicity and low cost enjoyed by a pure analog optical distribution scheme.
What is needed is a method for preserving the confidentiality of analog optical signals without using complex processing and expensive hardware. The present invention satisfies this need and provides additional advantages.